In the fabrication of integrated circuits, the exposure of a photoresist to light is an integral process step. Photoresists are utilized in microlithography processes for producing miniaturized electronic components, such as computer chips, semiconductors, integrated circuits, and the like. The production of such small-scale devices requires the application of a thin film coating of a photoresist material to a target substrate (i.e., silicon wafers). The coated substrate is then subjected to heat in order to drive off solvent within the photoresist and to fix the coating on the target substrate surface. The resultant coated substrate is then exposed to radiation (i.e., X-rays, visible light, ultraviolet light, electron beams, and the like) in order to etch certain images on the target substrate surface. A developer solution is then applied to the etched substrate to dissolve and remove either the untreated photoresist or the radiation-exposed photoresist.
Such a small-scale environment (for instance, these steps are performed in areas having submicron dimensions) requires that such radiation exposure be accomplished within close processing tolerances. For example, it is important to control the linewidth of the imaged and developed photoresist so that any deviation from the nominal design linewidth over non-planar or reflective features is small, typically less than 10%.
The difficulty in controlling linewidth in high resolution photoresist patterns over reflective topography is well documented. For example, see U.S. Pat. Nos. 5,731,385, to Knors et al., and 5,733,714, to McCulloch et al., for a detailed discussion of the processes and problems associated with photoresist etching in semiconductor chips and other such devices. All patent and prior art documents discussed within this specification are hereby incorporated by reference in their respective entireties. When photoresist layers overlaying reflective substrates are exposed using monochromatic light sources, a constructive interference pattern between the normally incident exposing light and light reflected from the substrate is created in the resist. The resulting pattern of optical nodes and antinodes which is normal to the plane of the reflective interface, and repeats along the optical axis, is the cause of localized variations in the effective dose of exposing light. This phenomenon is known in the art as the interference or standing wave effect. Other pattern distortions are caused by light reflected angularly from topographical features and are known in the art as reflective notching.
The quantification of the interference effect can be measured by using the swing ratio (SR), in accordance with the following equation: EQU SR=4(R.sub.1 R.sub.2).sup.1/2 e.sup.-.alpha.D
where R.sub.1 is the reflectivity of the resist/air interface and R.sub.2 is the reflectivity of the resist/substrate interface at the exposing wavelength, .alpha. is the resist absorption coefficient, and D is the resist thickness. A low swing ratio implies that localized variations in the effective dose of exposing light are small, and thus the exposure dose is more uniform throughout the thickness of the substrate. One method to reduce the swing ratio is to utilize a photoresist or lithographic process which imparts a high numerical value to the product of .alpha.D (i.e., increasing the light absorption by adding a colorant). Other methods include the utilization of coatings which reduce the contribution of R.sub.1 and/or R.sub.2, such as through the utilization of antireflective coating layers.
Antireflective coating (ARC) layers containing colorants permit the exposure imagewise of the photoresist through the exposure of the ARC layer which is applied on top of the photoresist. This coating is initially etched (which also etches the photoresist) and is thus transferred to the substrate surface. In such an instance, the etch rate of the ARC should be relatively high in order that the etching step may be performed without any appreciable loss of the photoresist film during the procedure. The colorants utilized in such ARC layers must not migrate from the ARC layer into the photoresist, must not sublime from the photoresist during the heating step (to evaporate solvent), and must provide an effective absorption the desired range of wavelengths.
Past antireflective coating layers have been developed which utilized various types of colorants for the absorption of errant and scattered light within certain wavelengths in order to provide a photoresist surface upon which proper etching with light may take place. Such colorants include polymeric dyes, diketo azo dyes, etc. Such dyes have proven effective in reducing the linewidth variations due to light reflections in non-uniform surfaces; however, these dyes also comprise a certain amount of unwanted and potentially deleterious metal counter ions, thereby easily crystallizing under certain conditions, and require the utilization of environmentally unfriendly solvents throughout the ARC layer-producing process. It has thus proven necessary to develop an antireflective coating comprising colorants which are water-soluble (and thus are relatively easy to handle and do not require the utilization of any potentially dangerous organic solvents), non-crystallizing, contain no metal counter ions, which exhibit good dry-etching properties, provide good image transfer from the photoresist to the substrate, and which are very effective in absorbing reflected light within certain specified wavelength ranges. The inventive antireflective coatings provide such beneficial characteristics.